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Abstract:

MgO-based magnetic tunnel junction (MTJ) device includes in essence a
ferromagnetic reference layer, a MgO tunnel barrier and a ferromagnetic
free layer. The microstructure of MgO tunnel barrier, which is prepared
by the metallic Mg deposition followed by the oxidation process or
reactive sputtering, is amorphous or microcrystalline with poor (001)
out-of-plane texture. In the present invention at least only the
ferromagnetic reference layer or both of the ferromagnetic reference and
free layer is proposed to be bi-layer structure having a crystalline
preferred grain growth promotion (PGGP) seed layer adjacent to the tunnel
barrier. This crystalline PGGP seed layer induces the crystallization and
the preferred grain growth of the MgO tunnel barrier upon post-deposition
annealing.

Claims:

1. A method of manufacturing a magnetic tunnel junction device, the
method comprising: a first layer deposition step of depositing a first
layer which contains ferromagnetic material; a tunnel barrier layer
deposition step of depositing a tunnel barrier layer onto the
ferromagnetic layer; and a second layer deposition step of depositing a
second layer which contains ferromagnetic material onto the tunnel
barrier layer, wherein the first layer deposition step comprises a first
amorphous ferromagnetic layer formation step of forming a first amorphous
ferromagnetic layer, and a second crystalline ferromagnetic layer
formation step of forming a second crystalline ferromagnetic layer, which
is the preferred grain growth promotion (PGGP) seed layer, sandwiched by
the first amorphous ferromagnetic layer and the tunnel barrier layer.

2. The method according to claim 1, wherein the first amorphous
ferromagnetic layer of the said first layer is ternary alloy containing
Co, Fe and B.

3. The method according to claim 1, wherein the first amorphous
ferromagnetic layer of the said first layer is formed with the thickness
between 1 nm and 4 nm.

4. The method according to claim 1, wherein the second crystalline
ferromagnetic layer, which is the PGGP seed layer, of the first layer is
formed using at least one of Co, Ni, Fe and B.

5. The method according to claim 1, wherein the second crystalline
ferromagnetic layer, which is the PGGP seed layer, of the first layer is
crystalline with a body-centered-cubic structure.

6. The method according to claim 1, wherein the second crystalline
ferromagnetic layer, which is the PGGP seed layer, of the first layer is
deposited with (001) out-of-plane texture.

7. The method according to claim 1, wherein the second crystalline
ferromagnetic layer, which is the PGGP seed layer, of the first layer is
formed with the thickness between 0.5 nm and 2 nm.

8. The method according to claim 1, wherein the thickness of the second
crystalline ferromagnetic layer, which is the PGGP seed layer, of the
first layer is equal to or less than the thickness of the thickness of
the first amorphous ferromagnetic layer of the first layer.

9. The method according to claim 1, wherein the tunnel barrier layer
deposition step comprises: a first metal layer formation step of forming
a first metal layer on the said second crystalline ferromagnetic layer,
which is the PGGP seed layer, and a process step of performing an
oxidation process through the said first metal layer to form a
metal-oxide layer, and a step of depositing a second metal cap layer onto
the metal-oxide layer.

10. The method according to claim 9, the oxidation process includes
radical oxidation, plasma oxidation, ozone oxidation and natural
oxidation.

11. The method according to claim 9, wherein the tunnel barrier is
amorphous or microcrystalline with poor (001) out-of-plane texture.

12. The method according to claim 1, wherein the tunnel barrier layer
deposition step comprises: a metal-oxide layer formation step of forming
a partly or fully oxidized metal-oxide layer on the second crystalline
ferromagnetic layer, which is the PGGP seed layer, a process step of
performing an oxidation process through the metal oxide layer to form a
fully oxidized metal-oxide layer, and a step of depositing a metal cap
layer onto the fully oxidized metal-oxide layer.

13. The method according to claim 12, the metal-oxide formation process
is reactive sputtering and the oxidation process includes radical
oxidation, plasma oxidation, ozone oxidation and natural oxidation.

14. The method according to claim 12, wherein the tunnel barrier prepared
by reactive sputtering is amorphous or microcrystalline with poor (001)
out-of-plane texture.

15. The method according to claim 1, wherein the second layer is an
amorphous layer, which is ternary alloy containing Co, Fe and B.

16. The method according to claim 1, wherein the second layer can also be
prepared by the deposition step which comprises: a third crystalline
ferromagnetic layer, which is the PGGP seed layer, formation step of
forming a third crystalline ferromagnetic layer deposited on the tunnel
barrier layer, and a fourth amorphous ferromagnetic layer formation step
of forming a fourth amorphous ferromagnetic layer deposited on the third
crystalline ferromagnetic layer, which is the PGGP seed layer.

17. The method according to claim 16, wherein the third crystalline
ferromagnetic layer, which is the PGGP seed layer, of the second layer is
formed using at least one of Co, Ni, Fe and B.

18. The method according to claim 16, wherein the third crystalline
ferromagnetic layer, which is the PGGP seed layer, of the second layer is
crystalline with a body-centered-cubic structure.

19. The method according to claim 16, wherein the third crystalline
ferromagnetic layer, which is the PGGP seed layer, of the second layer is
deposited with (001) out-of-plane texture.

20. The method according to claim 16, wherein the third crystalline
ferromagnetic layer, which is the PGGP seed layer, of the second layer is
formed with the thickness between 0.5 nm and 2 nm.

21. The method according to claim 16, wherein the fourth amorphous
ferromagnetic layer of the second layer is ternary alloy containing Co,
Fe and B.

22. The method according to claim 16, wherein the fourth amorphous
ferromagnetic layer of the second layer is formed with the thickness
between 1 nm and 4 nm.

23. The method according to claim 1, further comprising: a ferromagnetic
pinned layer deposition step of depositing a ferromagnetic pinned layer;
and a non-magnetic spacer layer deposition step of depositing a
non-magnetic spacer layer, wherein the first layer is deposited as a
ferromagnetic reference layer on the ferromagnetic pinned layer.

24. The method according to claim 23, wherein the ferromagnetic pinned
layer is amorphous ferromagnetic pinned layer, which is ternary alloy
containing Co, Fe and B.

25. The method according to claim 23, wherein the ferromagnetic pinned
layer can also be prepared by the deposition step comprises: a first
amorphous ferromagnetic pinned layer formation step of forming a first
amorphous ferromagnetic pinned layer, and a second crystalline
ferromagnetic pinned layer formation step of forming a second crystalline
ferromagnetic pinned layer deposited on the first amorphous ferromagnetic
pinned layer.

26. The method according to claim 25, wherein the second crystalline
ferromagnetic pinned layer of the said ferromagnetic pinned layer is
formed using at least one of Co, Ni, Fe and B.

27. The method according to claim 25, wherein the first amorphous
ferromagnetic pinned layer of the said ferromagnetic pinned layer is
ternary alloy containing Co, Fe and B.

28. The method according to claim 25, wherein the first amorphous
ferromagnetic pinned layer of the ferromagnetic pinned layer is formed
with the thickness between 0.5 nm and 2 nm.

29. The method according to claim 25, wherein the second crystalline
ferromagnetic pinned layer of the said ferromagnetic pinned layer is
formed with the thickness between 1 nm and 4 nm.

30. The method according to claim 1, further comprising; a process of the
crystallization of the first amorphous ferromagnetic layer and grain
growth of the tunnel barrier layer which is amorphous or microcrystalline
with poor (001) out-of-plane texture.

31. The method according to claim 30, wherein the process of the
crystallization of the first amorphous ferromagnetic layer and the
preferred grain growth of the said tunnel barrier layer is carried out
through vacuum annealing, and vacuum annealing condition is 300.degree.
C.˜400.degree. C., 1-4 hours under magnetic field of 3-10 kOe.

32. The method according to claim 30, wherein the crystallization of the
first amorphous ferromagnetic layer and the preferred grain growth of the
tunnel barrier layer is realized using the second crystalline
ferromagnetic layer, which is the PGGP seed layer, as adjacent
crystallization or grain growth seed layer during annealing, thus
eventually overall (001) out-of-plane texture of the first layer and
tunnel barrier layer is achieved.

33. A method of manufacturing a magnetic tunnel junction device, the
method comprising: a first layer deposition step of depositing a first
layer which contains ferromagnetic material, which comprises a first step
of forming a first amorphous ferromagnetic layer, and a second step of
forming a second crystalline layer on the first amorphous ferromagnetic
layer; a tunnel barrier layer deposition step of depositing a tunnel
barrier layer which is amorphous or microcrystalline with poor (001)
out-of-plane texture, and a process of crystallizing of the first layer
and grain growth of the tunnel barrier layer.

34. A method of manufacturing a magnetic tunnel junction device, the
method comprising: a first layer deposition step of depositing a first
layer which contains ferromagnetic material, which comprises a first step
of forming a first amorphous ferromagnetic layer, and a second step of
forming a second crystalline ferromagnetic layer on the first amorphous
ferromagnetic layer; a tunnel barrier layer deposition step of depositing
a tunnel barrier layer which consists of metal-oxide onto the lower
ferromagnetic layer, wherein the tunnel barrier layer deposition step
comprises a metal layer formation step of forming a metal layer onto the
second ferromagnetic layer, a process step of performing an oxidation
process through the metal layer to form a metal-oxide layer; and a
process of crystallizing or preferred grain growth of the first layer and
the tunnel barrier layer.

35. A method of manufacturing a magnetic tunnel junction device, the
method comprising: a first layer deposition step of depositing a first
layer which contains ferromagnetic material, which comprises: a first
step of forming a first amorphous ferromagnetic layer, and a second step
of forming a second crystalline ferromagnetic layer on the first
amorphous ferromagnetic layer; a tunnel barrier layer deposition step of
depositing a tunnel barrier layer which consists of metal-oxide onto the
lower ferromagnetic layer, wherein the tunnel barrier layer deposition
step comprises: a metal-oxide layer formation step of forming a partly or
fully oxidized metal-oxide layer on the second ferromagnetic layer, a
process step of performing an oxidation process through the metal oxide
layer to form a fully oxidized metal-oxide layer; and a process of
crystallizing or preferred grain growth of the first layer and the tunnel
barrier layer.

Description:

BACKGROUND ART

[0001] 1. Field of the Invention

[0002] The present invention is related to the field of magnetic tunnel
junction (MTJ) devices with particular reference to the magnetic random
access memory (MRAM) and the magnetic sensors including the recording
read head in hard disk drive and so on, which employ tunneling
magnetoresistance. More particularly, this invention relates to the MTJ
devices with MgO tunnel barrier prepared by oxidation methods or reactive
sputtering method, microstructure of which is amorphous or
microcrystalline with poor (001) out-of-plane texture. More particularly,
this invention relates to the MTJ devices with the insertion of the
crystalline ferromagnetic layers, which is the PGGP seed layers, adjacent
to the MgO tunnel barrier in order to enhance the crystallinity of the
MgO tunnel barrier during post-deposition annealing.

[0003] 2. Related Arts

[0004] Core element in the magnetic tunnel junction (MTJ) device is
"ferromagnetic layer/tunnel barrier/ferromagnetic layer" tri-layer
structure. The change of resistance of the MTJ device is attributed to
the difference in the tunneling probability of the spin polarized
electrons through the tunnel barrier on the bias voltage across the
device in accordance with the relative orientation of magnetizations of
the two ferromagnetic layers.

[0005] The relative orientation of the magnetizations of the two
ferromagnetic layers sandwiching the tunnel barrier is realized by the
different nature of the magnetization reversal of the two ferromagnetic
layers, in that the magnetization of one ferromagnetic layer is not
reversed by the external magnetic field in operation, whereas that of the
other ferromagnetic layer responds to the external magnetic field. Thus
parallel or antiparallel alignment of the magnetizations of the two
ferromagnetic layers sandwiching the tunnel barrier in device operation
is realized.

[0006] Tunnel barrier is commonly a dielectric material and must be ultra
thin and extremely uniform in thickness as well as composition. Any
inconsistency in terms of chemical stoichiometry or thickness degrades
the device performance significantly.

[0008] Ever since its discovery, high TMR at room temperature has been one
of hot topics of industries due to its spintronics application, such as
non-volatile magnetoresistive random access memory (MRAM) and magnetic
sensors such as the recording read-head in hard disk drive. For
conventional field switching MRAM application, 1 Mbit MRAM with the bit
size of 300×600 nm2 requires the MTJ to provide the
magnetoresistance (MR) ratio of 40% at the resistance-area (R×A)
product of about 1 k-2 k Ωμm2. At the higher density of 250
Mbits, the bit size scales down to 200×400 nm2 and requires MR
ratio of higher than 40% at the R×A product of about 0.5 k
Ωμm2. Further scaling can be achieved in MRAM by
application of magnetization reversal by the spin transfer torque,
however, it is required for the MTJ to provide MR ratio higher than 150%
at the R×A product range of 10-30 Ωμm2. For the
recording read-head in hard disk drive, it is required for the MTJ to
provide MR ratio higher than 50% at the R×A product range of 1-2
Ωμm2 in order to pick up reliable signals from the media
with areal density of 250 Gbit/in2.

[0009] Early efforts made on amorphous AlOx tunnel barrier and
ferromagnetic electrodes with high spin polarization were not
satisfactory for the requirements mentioned above. Recently single
crystal Fe/MgO/Fe has been suggested by theoretical calculation, (Butler
et al., Phys. Rev. B 63, (2001) p054416) and it is predicted that as high
as 6000% room temperature-TMR can be obtained due to a superior spin
filtering effect of MgO. This spin filtering effect, that is a total
reflection of minority spin down electrons in antiparallel magnetization
alignment of the two ferromagnetic layers sandwiching MgO tunnel barrier
of MTJ, is inherent from the absence of Bloch eigenstates in minority
spin-down spin channel with Δ1 symmetry at the Fermi surface. This
allows a coherent tunneling, and furthermore enables a giant TMR ratio.
There is a microstructural requirement to allow this coherent tunneling,
which is the epitaxial growth of Fe (001)/MgO (001)/Fe (001), in that the
tunneling electron passes through the (001) atomic planes of Fe and MgO.
Experimental attempts to achieve this giant TMR based on single crystal
(Fe/MgO/CoFe) growth using molecular beam epitaxy demonstrated room
temperature TMR up to 180%. (Yuasa et al. Appl. Phys. Lett. 87 (2005)
p222508) Using MgO tunnel barrier with polycrystalline CoFe ferromagnetic
electrodes, 220% room temperature TMR was reported, (Parkin et al. Nat.
Mater. 3 (2004) p862) and even higher TMR reported in MTJ prepared by
practical magnetron sputtering on thermally oxidized Si wafer using
amorphous CoFeB ferromagnetic electrodes. (Djayaprawira et al. Appl.
Phys. Lett. 86 (2005) p092502)

[0010] Great deal of efforts have been made to form the MgO tunnel barrier
in the MTJ, which is ultra thin and extremely uniform in thickness as
well as composition. Furthermore, similar amount of efforts have been
exerted to achieve the crystallinity of MgO tunnel barrier with (001)
out-of-plane texture in order to satisfy the microstrucural requirement,
(001) out-of-plane epitaxy together with bcc-structured sandwiching
ferromagnetic layers, given by the theoretical calculation and confirmed
by microstructural and thin film chemistry studies. (Y. S. Choi et al.
Appl. Phys. Lett. 90 (2007) p012505, Y. S. Choi et al. J. Appl. Phys. 101
(2007) p013907)

[0011] In general method of preparing MTJ devices for the mass production
of MRAM or recording read-head, the deposition of MgO tunnel barrier is
divided into the direct deposition and the metal deposition followed by
oxidation process. Deposition of tunnel barrier using ceramic target by
rf-sputtering or reactive sputtering of metal target in the ambience of
gas mixture of oxygen and inert gas falls into the first group of direct
deposition. Metal deposition followed by various kinds of oxidation
processes, such as natural oxidation, plasma oxidation, radical oxidation
or ozone oxidation, falls into the second group.

[0012] One of critical bottlenecks for MTJ development is the uniform
thickness control of tunnel barrier at the extremely thin thickness. If
the thickness of the tunnel barrier is too thin, it is highly possible to
contain pinholes, where leak current passes through without
spin-dependent tunneling. This degrades signal to noise ratio (S/N)
significantly. Another bottleneck is chemical inhomogeneity of tunnel
barrier, result in over- or under-oxidation, and the oxidation of
underlying ferromagnetic layer. These lead to asymmetric electrical
properties with respect to signs of applied bias and abnormal increase of
R×A product and decrease of TMR ratio due to the additional tunnel
barrier thickness with spin scattering in the surface-oxidized underlying
ferromagnetic layer. (Park et al. J. Magn. Magn. Mat., 226-230 (2001)
p926)

[0013] Besides the issues of the uniform thickness control of ultra-thin
MgO tunnel barrier and the chemical homogeneity across the MgO tunnel
barrier, most imminent issue to achieve the giant TMR ratio with low
R×A product of MgO-based MTJ is the (001) out-of-plane texture of
the ferromagnetic reference layer, MgO tunnel barrier and the
ferromagnetic free layer, and the high crystallinity of MgO tunnel
barrier. FIG. 2 shows the relationship of MgO texture and crystallinity
and the magnetotransport property in CoFeB/MgO/CoFeB MTJ, where MgO is
deposited by rf sputtering. It is clearly shown in the FIG. 2A and FIG.
2B that the MTJ prepared with highly crystalline and (001) textured MgO
tunnel barrier induces the corresponding (001) texture of CoFe through
crystallization of CoFeB amorphous layers by annealing, thus overall
(001) texture of CoFeB/MgO/CoFeB is realized. Therefore, it is possible
to obtain significantly enhanced MR ratio at low R×A product, as
shown in FIG. 2c. However, MTJ with MgO tunnel barrier with poor
crystallinity shows very low MR ratio with extremely high R×A
product, as also seen in FIG. 2c.

[0014] Despite MgO tunnel barrier prepared by rf sputtering has shown
great advances by process optimization, there are serious issues, which
are hard to overcome for the mass-production, in that MR ratio and
R×A product change very sensitively depending on the chamber
condition and particle generation inherent from rf-sputtering. (Oh et al.
IEEE Trans. Magn., 42 (2006) p2642) Furthermore, it has been reported
that the final R×A product uniformity (1σ) of MTJ devices
with MgO tunnel barrier prepared by rf-sputtering is more than 10%,
whereas that of MgO tunnel barrier prepared by Mg deposition followed by
oxidation process is less than 3%. (Zhao et al. US Patent Application, US
2007/0111332)

[0015] Alternative methods of MgO tunnel barrier preparation are the
metallic Mg deposition followed by the various oxidation processes or
reactive Mg sputtering in the ambience of gas mixture of oxygen and inert
gas. Plasma oxidation has been employed in the preparation for AlOx
tunnel barrier, however, its high reactivity makes it exceptionally
difficult to oxidize ultra-thin metal layer, especially very fast
oxidation rate of Mg for MgO formation, precisely down to the interface
with the underlying ferromagnetic layer. Thus R×A product and MR
ratio of 10000 Ωμm2/45% are obtained by plasma oxidation
process, (Tehrani et al. IEEE Trans. Magn., 91 (2003) p703) whereas those
of 1000 Ωμm2/30% by ozone oxidation from MTJ with AlOx
tunnel barrier. (Park et al. J. Magn. Magn. Mat., 226-230 (2001) p926)

[0016] Therefore, less energetic oxidation processes have been suggested,
which are radical oxidation and natural oxidation to form MgO tunnel
barrier. Also reactive sputtering of Mg metal target to form MgO tunnel
barrier in the ambience of Ar and O2. FIG. 3 shows the
magnetotransport property measurement results obtained from MTJs with MgO
tunnel barrier prepared by various methods of MgO tunnel barrier
deposition. The MTJ structure is identical except the MgO tunnel barrier
part, which is bottom layers/PtMn (15)/CoFe (2.5)/Ru (0.9)/CoFeB (3)/MgO
(x)/CoFeB (3)/capping layer. Thickness in parenthesis is in nanometer
scale. With reference to the MR ratio and R×A product obtained from
the MTJ with MgO prepared by rf sputtering, it is clearly shown that the
MR ratio of the MTJ with MgO tunnel barrier prepared by oxidation methods
and reactive sputtering is significantly lower. At given R×A
product of 10 Ωμm2, MTJ with MgO prepared by rf sputtering
provides MR ratio of 180%, whereas MgO deposited by radical oxidation
method provides 100%, natural oxidation provides 60%, and MgO prepared by
reactive sputtering provides 135%.

[0017] The microstructure analyses were carried out with high-resolution
transmission microscopy (HREM) and x-ray diffraction (XRD) and x-ray
photoelectron spectroscopy (XPS). As shown in FIG. 4A and FIG. 4B, it is
clearly compared that the difference in the magnetotransport property
results from the difference in the crystallinity of MgO tunnel barrier
and the lack of epitaxy in CoFeB/MgO/CoFeB layers. FIG. 4A and FIG. 4B
are cross-section HREM images taken from the MTJs with MgO tunnel barrier
prepared by rf-sputtering and radical oxidation, respectively. As
reported by Choi et al. in J. Appl. Phys. 101 (2007) p013907,
CoFeB/MgO/CoFeB-based MTJ prepared by rf sputtering satisfies the
microstructural requirement given by the theoretical calculations by
Butler et al., in that MgO is highly crystalline and in good
grain-to-grain epitaxy with CoFe layers. The CoFe layers are crystallized
by post-deposition annealing based on the crystalline MgO as a
crystallization template, thus the grain-to-grain epitaxy is realized in
CoFe/MgO/CoFe layers. However, MgO tunnel barrier prepared by radical
oxidation shows the poor crystallinity mixed with amorphous and it is
hard to confirm the pseudo-epitaxy at the interface with CoFe layers.

[0018] FIG. 4C shows the clear comparison of the MgO crystallinity and
texture with respect to its deposition method, rf-sputtering and natural
oxidation. Out-of-plane theta-2 theta scan confirms that MgO tunnel
barrier deposited on the amorphous CoFeB layer by rf-sputtering is highly
crystalline in as-grown state and highly textured with (001) out-of-plane
preferred orientation by pronounced MgO (002) peak at 2
theta=42.4°. However, MgO prepared by metal deposition followed by
natural oxidation shows no pronounced peak, which indicates that the MgO
layer is almost amorphous.

[0019]FIG. 4D and FIG. 4E are XPS spectra obtained from the MTJs with MgO
tunnel barrier prepared by rf-sputtering and reactive sputtering,
respectively. As reported by Choi et al. in Appl. Phys. Lett. 90 (2007)
p012505, it is critical to have the dominant population of oxygen ions in
the lattice point of NaCl-structured MgO for the crystallinity of MgO and
higher MR ratio of the MTJ and lower R×A product. It is clear, as
shown in FIG. 4D, that the population of oxygen ions (whose binding
energy is around 531 eV) occupying lattice point of NaCl-structured MgO
is very high in the MgO deposited by rf sputtering, however, there is
considerable population of impurity oxygen ion (whose binding energy is
around 533.3 eV), as shown in FIG. 4B, which is almost a third of that of
oxygen ion in the lattice point in the MgO deposited by reactive
sputtering. Thus it can be inferred that this high density of impurity
oxygen ions in the MgO barrier is related to the poor crystallinity of
MgO and is responsible for the poor MR ratio.

[0020] In order to achieve good crystallinity of MgO tunnel barrier
prepared by oxidation method, crystalline ferromagnetic reference layer,
not bi-layer but single layer, has been employed, in that the structure
of MTJ is bottom layers/PtMn (15)/CoFe (2.5)/Ru (0.9)/CoFe (3)/MgO
(x)/CoFeB (3)/capping layer. As shown in FIG. 5A, MTJ with fully
crystalline CoFe single reference layer provides noticeable drop of MR
ratio to 35% from 130% by CoFeB amorphous reference layer. And the shape
of full hysteresis loop, FIG. 5B, from MTJ with fully crystalline CoFe
single reference layer after as-deposition annealing at 360° C.
for 2 hrs under 10 kOe magnetic field indicates that the poor or
destroyed SAF structure, whereas that of MTJ, as shown in FIG. 5c, with
amorphous CoFeB single reference layer after same condition of
post-deposition annealing shows clear SAF coupling in the circle mark.
Body-centered-cubic CoFe tends to grow (110) atomic planes parallel to
the interface with Ru in order for the lattice match with
hexagonal-close-packed Ru (0001) basal plane. (110) out-of-plane texture
of ferromagnetic reference layer is not preferable for the giant TMR from
the theoretical calculation by Butler et al. in MgO-based MTJ.
Furthermore, the thermal stability of SAF(CoFeB/Ru/CoFe) is much worse
than that of SAF(CoFeB/Ru/CoFeB), thus clearly distinctive magnetization
separation between constituent ferromagnetic layers cannot be secured if
the MTJ is composed of CoFeB/Ru/CoFe SAF structure after high temperature
post-deposition annealing. Thus the crystalline CoFe single reference
layer is proven to be not effective to achieve the good crystallinity of
MgO tunnel barrier.

[0021] Consequently, it can be understood that the poor crystallinity of
MgO tunnel barrier deposited by oxidation method or reactive sputtering
cannot play a role of crystallization template to crystallize amorphous
CoFeB into CoFe at the CoFeB/MgO interface. Thus no grain-to-grain
pseudo-epitaxy can be expected in CoFe/MgO/CoFe layers, which results in
the poor magnetotransport property.

[0034] The objective of the present invention is to provide satisfactory
high MR ratio at low R×A product for the application to the spin
transfer torque MRAM and the recording read-head from the MTJ with MgO
tunnel barrier, which is prepared by the metal deposition followed by the
various oxidation methods or prepared by the reactive sputtering and the
microstructure of which is amorphous or microcrystalline tunnel barrier
with poor (001) out-of-plane texture.

[0035] According to a first aspect of the present invention, it is
critical to crystallize or induce the preferred grain growth in the MgO
tunnel barrier prepared by the metal deposition followed by the various
oxidation methods or prepared by the reactive sputtering.

[0036] According to a second aspect of the present invention, the
crystallization or the preferred grain growth of the MgO tunnel barrier,
which is amorphous or microcrystalline with poor (001) out-of-plane
texture in as-grown state, can be achieved during the post-deposition
annealing by use of crystalline ferromagnetic PGGP seed layer with
body-centered-cubic structure under or sandwiching the MgO tunnel
barrier.

[0037] According to a third aspect of the present invention, the
microstructure of MTJ with MgO tunnel barrier after post-deposition
annealing is eventually overall (001) out-of-plane texture of the
ferromagnetic reference layer, MgO tunnel barrier and the ferromagnetic
free layer.

[0038] According to a forth aspect of the present invention, the MTJ
device includes an antiferromagnetic pinning layer, a synthetic
antiferromagnetic pinned layer, a tunnel barrier and a ferromagnetic free
layer. The synthetic antiferromagnetic pinned layer includes a
ferromagnetic pinned layer, a non-magnetic spacer and a ferromagnetic
reference layer.

[0039] It is preferred that the ferromagnetic reference layer is formed in
bi-layer structure, in that the first amorphous ferromagnetic reference
layer deposited on the non-magnetic spacer and the second crystalline
ferromagnetic reference layer, which is PGGP seed layer, deposited on the
said first amorphous ferromagnetic reference layer.

[0040] It is preferred that the first amorphous ferromagnetic reference
layer in the bi-layer-structured ferromagnetic reference layer is the
ternary alloy containing Co, Fe and B, in which the content of boron is
higher than 12 atomic %.

[0041] It is preferred that the thickness of the first amorphous
ferromagnetic reference layer in the bi-layer-structured ferromagnetic
reference layer is between 1 nm to 4 nm.

[0042] It is preferred that the second crystalline ferromagnetic reference
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
reference layer is the binary alloy of CoxFe100-x, in which 0<x<80.

[0043] It is preferred that the second crystalline ferromagnetic reference
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
reference layer can also be formed by single Fe element.

[0044] It is preferred that the second crystalline ferromagnetic reference
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
reference layer can also be the ternary alloy containing Co, Fe and B, in
which the content of boron is less than 12 atomic %, thus crystalline
ternary alloy whose content of boron is less than 12 atomic %.

[0045] It is preferred that the thickness of the second crystalline
ferromagnetic reference layer in the bi-layer-structured ferromagnetic
reference layer is between 0.5 nm to 2 nm.

[0046] It is preferred that the thickness of the second crystalline
ferromagnetic reference layer in the bi-layer-structured ferromagnetic
reference layer is equal to or less than the thickness of the said first
amorphous ferromagnetic reference layer in the bi-layer-structured
ferromagnetic reference layer.

[0047] It is preferred that the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, is prepared by the
deposition of metallic Mg layer, the oxidation of the said metallic Mg
layer by radical oxidation, plasma oxidation, natural oxidation or ozone
oxidation, then finally deposition of the metallic Mg cap layer after
oxidation.

[0048] It is preferred that the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, can also be
prepared by the deposition of partially or fully oxidized Mg-oxide layer
using reactive sputtering, the oxidation of the said partially or fully
oxidized Mg-Oxide layer by radical oxidation, plasma oxidation, natural
oxidation or ozone oxidation, then finally metallic Mg cap layer after
oxidation.

[0049] It is preferred that the ferromagnetic free layer also can be
formed in bi-layer structure, in that the first crystalline ferromagnetic
free layer, which is PGGP seed layer, deposited on the MgO tunnel barrier
and the second amorphous ferromagnetic free layer deposited on the said
first crystalline ferromagnetic free layer.

[0050] It is preferred that the first crystalline ferromagnetic free
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
free layer is the binary alloy of CoxFe100-x, in which 0<x<80.

[0051] It is preferred that the first crystalline ferromagnetic free
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
free layer can also be formed by single Fe element.

[0052] It is preferred that the first crystalline ferromagnetic free
layer, which is PGGP seed layer, in the bi-layer-structured ferromagnetic
free layer can also be the ternary alloy containing Co, Fe and B, in
which the content of boron is less than 12 atomic %, thus crystalline
ternary alloy whose content of boron is less than 12 atomic %.

[0053] It is preferred that the thickness of the first crystalline
ferromagnetic free layer, which is PGGP seed layer, in the
bi-layer-structured ferromagnetic free layer is between 0.5 nm to 2 nm.

[0054] It is preferred that the second amorphous ferromagnetic free layer
in the bi-layer-structured ferromagnetic free layer is the ternary alloy
containing Co, Fe and B, in which the content of boron is higher than 12
atomic %.

[0055] It is preferred that the thickness of the second amorphous
ferromagnetic free layer in the bi-layer-structured ferromagnetic free
layer is between 1 nm to 4 nm.

[0056] It is preferred that the magnetic tunnel junction device with
crystalline ferromagnetic layer inserted between the amorphous
ferromagnetic layer and the amorphous or microcrystalline MgO tunnel
barrier with poor (001) out-of-plane texture shows significantly reduced
resistance-area product and noticeably increased magnetoresistance ratio
compared to the magnetic tunnel junction device without the insertion of
the crystalline ferromagnetic layer, which is the PGGP seed layer,
inserted between the amorphous ferromagnetic layer and the amorphous or
microcrystalline MgO tunnel barrier with poor (001) out-of-plane texture.

[0057] It is preferred that the resistance-area product and the
magnetoresistance ratio of the magnetic tunnel junction device with
crystalline ferromagnetic layer, which is the PGGP seed layer, inserted
between the amorphous ferromagnetic layer and the amorphous or
microcrystalline MgO tunnel barrier with poor (001) out-of-plane texture
are less than 5 Ωμm2 and higher than 170%, respectively.

[0058] The existence of the second crystalline ferromagnetic reference
layer and/or the said first crystalline ferromagnetic free layer, which
are PGGP seed layers, induce the crystallization and the preferred grain
growth of the MgO tunnel barrier, which is amorphous or microcrystalline
tunnel barrier with poor (001) out-of-plane texture in as-deposited
state, after post-deposition annealing, as schematically described in
FIG. 6A.

[0059] Also the existence of the second crystalline ferromagnetic
reference layer and/or the said first crystalline ferromagnetic free
layer, which are PGGP seed layers, induce the crystallization and the
preferred grain growth of the said first ferromagnetic amorphous
reference layer and/or the said second ferromagnetic amorphous free
layer, which are amorphous in as-deposited state, after post-deposition
annealing, as schematically described in FIG. 6A.

[0060] Therefore, the microstructure of MTJ after post-deposition
annealing is eventually overall (001) out-of-plane texture of the
ferromagnetic reference layer, MgO tunnel barrier and the ferromagnetic
free layer. With this obtained microstructure of MTJ of the present
invention, it is possible to achieve the significant increase of MR ratio
as well as the noticeable reduction of R×A product, as shown in
FIG. 6A. However, optimum annealing temperature of MTJ with
bi-layer-structured ferromagnetic reference layer and/or
bi-layer-structured ferromagnetic reference layer of the present
invention cannot be lower than that of MTJ with rf-sputtered crystalline
MgO and the single-layered amorphous ferromagnetic reference layer and
free layer. It is easy to explain this increase of optimum annealing
temperature by the difference of crystallization object. The layers to be
crystallized in the MTJ of the present invention are the MgO tunnel
barrier and the amorphous ferromagnetic layers, as shown in FIG. 6A,
whereas the layers to be crystallized in the MTJ with rf-sputtered
crystalline MgO are the amorphous ferromagnetic layers only, as shown in
FIG. 6B. As the melting point of CeFe is much lower than that of MgO
(thus the recrystallization temperature would correspond accordingly), it
can be intuitively known that the temperature required to crystallize the
CoFe would be lower than that for MgO.

[0061] Similar structure of bi-layered ferromagnetic reference layer has
been suggested by Miura et al. in Japanese patent application JP
2008-135432, which suggests the insertion of the amorphous or
microcrystalline CoFe layer between the amorphous ferromagnetic CoFeB
layer and the crystalline MgO tunnel barrier. It is claimed that the
insertion of amorphous or microcrystalline CoFe layer effectively lowers
the annealing temperature down to 300° C. by the crystallization
template effect of the crystalline MgO tunnel barrier. However, this is
not applicable to the MTJ with MgO tunnel barrier prepared by oxidation
methods or the reactive sputtering as the MgO tunnel barrier is amorphous
or microcrystalline with poor (001) out-of-plane texture in as-deposited
state.

[0062] Also Nishimura et al. (Patent Reference 3) has suggested the
identical bi-layered ferromagnetic layer, which is the insertion of the
amorphous or microcrystalline CoFe layer between the amorphous
ferromagnetic CoFeB layer and the crystalline MgO tunnel barrier under
the MgO tunnel barrier in the Japanese patent application JP 2008-103661.
Despite this patent application covers the MgO preparation methods
including rf sputtering and oxidation methods, only rf-sputtered MgO
tunnel barrier, which is highly probable to be good crystalline, is
suggested in the preferred embodiment and the MgO tunnel barrier
deposited by reactive sputtering is not included. As mentioned above, it
is hard to apply same argument of using the crystalline MgO as a
crystallization template for high MR ratio to the MTJ with MgO tunnel
barrier which is amorphous or microcrystalline with poor (001)
out-of-plane texture in as-deposited state.

BRIEF DESCRIPTION OF DRAWINGS

[0063]FIG. 1 is a schematic of a typical structure of the magnetic tunnel
junction.

[0070]FIG. 8A and FIG. 8B show schematics of MTJs in the first and second
embodiments of the invention with CoFe PGGP seed layer (FIG. 8A) only in
reference layer and (FIG. 8B) in both of reference and free layer.

[0071]FIG. 8c shows schematic of MTJ in the third embodiment of the
invention.

[0072] FIG. 8D shows schematic of MTJ in the forth embodiment of the
invention.

[0073] FIG. 9A shows the magnetotransport property comparison of A and B
MTJ stacks of the first embodiment of the present invention compared to
that of MTJs with MgO prepared by rf-sputtering and radical oxidation
without PGGP seed layer and FIG. 9B shows MR ratio and R×A product
comparison of A and B MTJ stacks of the first embodiment with respect to
the reference identical MTJ stack only without PGGP seed layer.

[0074]FIG. 10A shows the magnetotransport property comparison of A and B
MTJ stacks of the second embodiment of the present invention compared to
that of MTJs with MgO prepared by rf-sputtering and radical oxidation
without PGGP seed layer and FIG. 10B shows MR ratio and R×A product
comparison of A and B MTJ stacks of the second embodiment with respect to
the reference identical MTJ stack only without PGGP seed layer.

[0075] FIG. 11 shows MR ratio and R×A product comparison of A MTJ
stack of the second embodiment, of which the thickness of the first
amorphous CoFeB ferromagnetic reference layer is fixed at 1.5 nm, whereas
the thickness of the second crystalline CoFe ferromagnetic reference
layer, which is PGGP seed layer, varies.

[0076] FIG. 12 shows MR ratio and R×A product comparison of A MTJ
stack of the third embodiment with respect to the reference identical MTJ
stack only without PGGP seed layer.

[0077] FIG. 13 shows MR ratio and R×A product comparison of A MTJ
stack of the forth embodiment with respect to the reference identical MTJ
stack only without PGGP seed layer.

[0078] FIG. 14 shows schematic of MTJ of the fifth embodiment in the
present invention with CoFeB PGGP seed layer in both of reference layer
and free layer.

[0079] FIG. 15A and FIG. 15B show the XRD theta-2 theta scan obtained from
CoFeB single layers, whose boron contents are 20 atomic %, 5.1 atomic %
and 2.9 atomic %. Peak position, full width at half maximum and the
resistivity from each of single layers are shown in the table.

[0080]FIG. 16A shows the magnetotransport property comparison of A and B
MTJ stacks of the fifth embodiment of the present invention compared to
that of MTJs with MgO prepared by rf-sputtering and radical oxidation
without PGGP seed layer.

[0081] FIG. 16B shows MR ratio and R×A product comparison of A and B
MTJ stacks of the second embodiment with respect to the reference
identical MTJ stack only without PGGP seed layer.

[0082] FIG. 17 shows schematic of MTJ of the sixth embodiment in the
present invention with Fe PGGP seed layer in both of reference layer and
free layer.

[0083] FIG. 18 shows schematic of MTJ of the seventh embodiment in the
present invention with CoFe PGGP seed layer in both of reference layer
and free layer. Also bi-layer structured ferromagnetic pinned layer is
employed.

[0084]FIG. 19 shows the magnetotransport property comparison of A MTJ
stack of the seventh embodiment of the present invention compared to that
of MTJ with MgO prepared by natural oxidation without PGGP seed layer.

[0093]FIG. 7 exemplifies a MTJ device manufacturing apparatus in the
preferred embodiment. FIG. 7 is a schematic plan view of a vacuum
processing system 700 for fabricating a magnetic tunnel junction device.
A vacuum processing system shown in FIG. 7 is a cluster type system
providing a plurality of thin film deposition chambers using physical
vapor deposition technique. Plurality of deposition chambers in the said
vacuum processing system is attached to the vacuum transfer chamber 701
provided with robot loaders at the center position (not shown). The said
vacuum processing system 700 is equipped with two load-lock chambers 702
and 703 to load/unload substrates. The said vacuum processing system is
equipped with degas chamber 704 and pre-etch/etch chamber 705. The vacuum
processing system is equipped with oxidation chamber 706 and plurality of
metal deposition chambers 707, 708, and 709. Each of chambers in the
vacuum processing system is connected through a gate valve in order to
open/close the passage between the chambers. Note that each of chambers
in the vacuum processing system is equipped with pumping system, gas
introduction system, and power supply system. Moreover, the gas
introduction system comprises flow-regulating means, the pumping system
comprises pressure regulating means. Each operation of the flow
regulating means and the pressure regulating means can control a certain
pressure in the chamber during a certain period of time. Moreover,
operations based on combination of the flow regulating means and the
pressure regulating means can control the certain pressure in the chamber
during the certain period of time.

[0094] In each of the metal deposition chambers 707, 708, and 709 of the
said vacuum processing system 700, each of the magnetic layers and the
non-magnetic metal layers is deposited on the substrate one by one by the
sputtering method. In the metal deposition chambers 707, 708, and 709,
for example, a material of a target is "CoFe", a material of a target is
"Ru", a material of a target is "CoFeB", and a material of a target is
"Mg". And a material of a target is "antiferromagnetic material", a
material of a target is "seed material", a material of a target is
"capping material". Furthermore, a material of a target is "top electrode
material" and a material of a target is "bottom electrode material".
Pre-etching and etching are carried out in the pre-etch/etch chamber.
Oxidation is carried out in the oxidation chamber 706. Moreover, each
metal deposition chamber comprises a sputtering apparatus which can
perform dc-sputtering. Procedures, such as gas introduction into each
chamber, switching the valve, power supply ON/OFF, an exhaust gas, and a
substrate transfer, is carried out by a system controller (not shown).

[0095]FIG. 1 shows a typical stack structure 100 of MTJ for tunneling
magnetoresistance (TMR) sensor or memory cell. Most advantageously, on an
under layer 102 and Si wafer 101 MTJ is composed of an antiferromagnetic
pinning layer 103, a synthetic antiferromagnetic (SAF) pinned layer 110,
a tunnel barrier 107 and a ferromagnetic free layer 108. A capping layer
109 on which top electrode 110 is attached is formed on the free layer
108. In the stack structure shown in FIG. 1, a synthetic
antiferromagnetic pinned layer 110 is formed by including a ferromagnetic
pinned layer 104, a non-magnetic spacer 105 and a ferromagnetic reference
layer 106.

[0096] The MTJ devices of the present invention are formed by preparation
of the core element in the MTJ device, the core comprising "ferromagnetic
pinned layer 104/non-magnetic spacer 105/ferromagnetic reference layer
106/tunnel barrier 107/ferromagnetic free layer 108" multilayer
structure, using the combinations of materials selected from the
following groups for the preferred embodiments.

[0114] The first embodiment is a method of forming the tunnel barrier of
MTJ devices by radical oxidation method and the employment of CoFe as the
preferred grain growth promotion seed layer, in that the core element of
the MTJ is formed by the combination of (a+d+i+k) or (a+d+j+k) of the
group 1, 2, 3 and 4 mentioned above.

[0115] Two different configurations of MTJ stacks, as illustrated in FIG.
8A and FIG. 8B are used in the first embodiment as follows;

[0116] Referring to FIG. 8A and FIG. 8B, one of important aspect of the
first embodiment is the insertion of the preferred grain growth promotion
(PGGP) seed layer, which is the second crystalline CoFe ferromagnetic
reference layer 807-2, 807-2' and/or the first crystalline CoFe
ferromagnetic free layer 807-1', under or sandwiching the MgO tunnel
barrier.

deposition of first metallic Mg layer 808, 808' on the second crystalline
ferromagnetic reference layer 807-2, 807-2', which is the preferred grain
growth promotion seed layer, with thickness of 1.1 nm, oxidation of the
first metallic layer 808, 808' by radical oxidation carried out in the
oxidation chamber, in which electrically-ground "shower plate" is placed
between an upper ionizing electrode and the substrate. Oxygen plasma is
generated by applying 300 W of rf power to the ionizing electrode with
oxygen flow of 700 sccm. Oxygen radical shower flows through the shower
plate, whereas particles with electric charge, such as ionized species
and electrons, cannot pass through due to the electric grounding of
shower plate, and deposition of the metallic Mg cap layer 810, 810' with
thickness of 0.3 nm on the first metallic Mg layer oxidized by radical
oxidation.

[0120] Also with reference to FIG. 8B, the first crystalline ferromagnetic
Co(70 at. %)Fe(30 at. %) free layer 807-1', which is the preferred grain
growth promotion seed layer, is deposited with thickness of 1.5 nm on the
metallic Mg cap layer 810'. Then the second amorphous Co(60 at. %)Fe(20
at. %)B(20 at. %) ferromagnetic free layer 811' with thickness of 1.5 nm
is deposited on the first crystalline ferromagnetic free layer 807-1'.

[0121] Post-deposition magnetic field annealing is carried out at
360° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer 806, 806' and/or the second amorphous
ferromagnetic free layer 811, 811' and the preferred grain growth of the
said amorphous or microcrystalline MgO tunnel barrier with poor (001)
out-of-plane texture. This crystallization and the preferred grain growth
are realized using the second crystalline CoFe ferromagnetic reference
layer 807-2, 807-2' and/or the first crystalline CoFe ferromagnetic free
layer 807-1' as adjacent crystallization or preferred grain growth seed
layer during annealing, thus eventually overall (001) out-of-plane
texture of the ferromagnetic reference layer, MgO tunnel barrier and the
ferromagnetic free layer.

[0122] With reference to FIG. 9A and FIG. 9B, the magnetotransport
properties of the MTJ prepared by the method of the present invention
were measured using CIPT method. In order for comparison, the MTJ with
MgO tunnel barrier prepared by rf sputtering and the MTJ with MgO tunnel
barrier prepared by same oxidation method, in both of which crystalline
CoFe PGGP seed layer is not employed, are shown as reference. As shown in
FIG. 9A, with reference to the MR ratio and R×A product obtained
from the MTJ with MgO prepared by same oxidation method but without the
insertion of the crystalline PGGP seed layer, it is apparent that the MTJ
which employs the crystalline PGGP layer shows much improved
magnetotranport properties, in that noticeable increase of MR ratio is
obtained with significant reduction of R×A product, which is
comparable to or even better than those from MTJ with MgO tunnel barrier
prepared by rf sputtering. At given R×A product of about 9
Ωμm2, MTJ with MgO prepared by radical oxidation without
the crystalline CoFe PGGP seed layer provides MR ratio of 103%, whereas
the MTJs with MgO deposited by radical oxidation with the crystalline
CoFe PGGP seed layer, A stack and B stack, provide 200% and 190%,
respectively, which are comparable to or even higher than 185% obtained
from the MTJ with MgO tunnel barrier prepared by rf sputtering without
the crystalline PGGP seed layer. Clear evidence of R×A product
reduction and MR ratio increase is shown in FIG. 9B. Only difference
between reference and A and B is whether the MTJ employs the crystalline
PGGP seed layer or not. The radical oxidation condition for all samples
are identical; 300 W, 700 sccm and 10 seconds. By use of the crystalline
PGGP seed layer, R×A product drops from 14 Ωμm2 to
7.5 Ωμm2, which roughly drops by half, and MR ratio
increases from 110% to 180%.

[0123] Based on the previous studies correlating magnetotransport property
and the crystallinity and pseudo-epitaxy in MTJ, it can be intuitively
inferred that the insertion of the crystalline CoFe PGGP seed layer
adjacent the MgO tunnel barrier, which is amorphous or microcrystalline
with poor (001) out-of-plane texture, induces the crystallization and the
preferred grain growth of the MgO tunnel barrier during the
post-deposition annealing.

Second Embodiment

[0124] The second embodiment is a method of forming the tunnel barrier of
MTJ devices by natural oxidation method and the employment of CoFe as the
preferred grain growth promotion seed layer, in that the core element of
the MTJ is formed by the combination of (a+d+i+k) or (a+d+j+k) of the
group 1, 2, 3 and 4 mentioned above.

[0125] Two different configurations of MTJ stacks, as illustrated in FIG.
8A and FIG. 8B, are used in the second embodiment as follows;

[0126] Referring to FIG. 8A and FIG. 8B, which also illustrate
configurations of the stacks A and B of the second embodiment, one of
important aspect of the second embodiment is the insertion of the
preferred grain growth promotion seed layer, which is the second
crystalline CoFe ferromagnetic reference layer 807-2, 807-2' and/or the
first crystalline CoFe ferromagnetic free layer 807-1', under or
sandwiching the MgO tunnel barrier.

deposition of first metallic Mg layer 808, 808' on the second crystalline
ferromagnetic reference layer 807-2, 807-2', which is the preferred grain
growth promotion seed layer, with thickness of 1.1 nm, oxidation of the
first metallic layer 808, 808' by natural oxidation carried out in the
oxidation chamber. The natural oxidation process, which is advantageously
applied to the thinly formed metallic Mg layer, requires purging the
oxidation chamber with oxygen gas at a pressure of approximately
6.5×10-1 Pa and flowing the oxygen gas at the flow rate of 700
sccm, then leaving the as-deposited metallic Mg layer in contact with the
oxygen gas flow for given exposure time, and deposition of the metallic
Mg cap layer 810, 810' with thickness of 0.3 nm on the first metallic Mg
layer 809, 809' oxidized by natural oxidation.

[0130] Also with reference to FIG. 8B, the first crystalline ferromagnetic
Co(70 at. %)Fe(30 at. %) free layer 807-1', which is the preferred grain
growth promotion seed layer, is deposited with thickness of 1.5 nm on the
metallic Mg cap layer 810'. Then the second amorphous Co(60 at. %)Fe(20
at. %)B(20 at. %) ferromagnetic free layer 811' with thickness of 1.5 nm
is deposited on the first crystalline ferromagnetic free layer 807-1'.

[0131] With reference to FIGS. 20A to 20C and FIG. 21, it is clear that
the microstructure of the as-grown second CoFe ferromagnetic reference
layer 807-2, 807-2', which is the preferred grain growth promotion seed
layer, on the first amorphous CoFeB ferromagnetic reference layer 806,
806' is crystalline with body-centered-cubic structure and with (001)
out-of-plane texture. FIGS. 20A to 20C show the method of analysis of
cross-section image obtained by the high-resolution transmission electron
microscope to confirm whether CoFe PGGP seed layer 807-2, 807-2' grows
with (001) out-of-plane or (011) out-of-plane. Inter-atomic spacing (d)
of CoFe PGGP seed layer 807-2, 807-2' sandwiched by the MgO tunnel
barrier and the first amorphous CoFeB ferromagnetic reference layer 806,
806' in FIG. 20A is d110 when the CoFe PGGP seed layer 807-2, 807-2'
deposited on the first amorphous CoFeB ferromagnetic reference layer 806,
806' grows with (001) out-of-plane as shown in FIG. 20B, whereas the
inter-atomic spacing (d) is d200 when the CoFe PGGP seed layer
807-2, 807-2' deposited on the first amorphous CoFeB ferromagnetic
reference layer 806, 806' grows with (011) out-of-plane as shown in FIG.
20C. Inter-atomic spacing of (110) atomic planes (d110) of the CoFe
with body-centered-cubic structure is 2.02 Å and d200 is 1.41
Å. With respect to FIG. 21, overall crystallinity of the second CoFe
ferromagnetic reference layer 807-2, 807-2', which is the preferred grain
growth promotion seed layer, is confirmed. Inter-atomic spacing of CoFe
PGGP seed layer 807-2, 807-2' is measured using d111 of Cu layer as
a reference, where d111 is 2.08 Å, for the length reference (not
shown here). Using this reference, the inter-atomic spacing of CoFe PGGP
seed layer 807-2, 807-2' was measured by averaging 6 atomic planes, which
provides the inter-atomic spacing is 2.02 Å. Therefore, it can be
confirmed that the crystalline CoFe PGGP seed layer 807-2, 807-2' on the
first amorphous CoFeB ferromagnetic reference layer 806, 806' grows with
(001) out-of-plane. Furthermore, it also can be confirmed that the
thickness of MgO tunnel barrier is 5 mono-layers, which is 10.5 Å
correspondingly, and clear partition of the first amorphous CoFeB
ferromagnetic reference layer 806, 806' and the second crystalline CoFe
ferromagnetic reference layer 807-2, 807-2'.

[0132] Post-deposition magnetic field annealing is carried out at
360° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer 806, 806' and/or the second amorphous
ferromagnetic free layer 811, 811' and the preferred grain growth of the
said amorphous or microcrystalline MgO tunnel barrier with poor (001)
out-of-plane texture. This crystallization and the preferred grain growth
are realized using the second crystalline CoFe ferromagnetic reference
layer 807-2, 807-2' and/or the first crystalline CoFe ferromagnetic free
layer 807-1' as adjacent crystallization or preferred grain growth seed
layer during annealing, thus eventually overall (001) out-of-plane
texture of the ferromagnetic reference layer, MgO tunnel barrier and the
ferromagnetic free layer.

[0133] With reference to FIG. 22A and FIG. 22B, it is clear that the
microstructure of the second CoFe ferromagnetic reference layer 807-2,
807-2', which is the preferred grain growth promotion seed layer, and the
first CoFeB ferromagnetic reference layer 806, 806' form a
single-layer-structured fully crystalline CoFe reference layer with
body-centered-cubic structure and with (001) out-of-plane texture. The
same argument to analyze the cross-section image (FIGS. 20A to 20C)
obtained by the high-resolution transmission electron microscope is
applied to confirm whether single-layer-structured CoFe reference layer
after annealing is crystalline with (001) out-of-plane or (011)
out-of-plane. With respect to FIG. 22A, the single-layered structure of
reference layer is confirmed to be formed by merging the second CoFe
ferromagnetic reference layer 807-2, 807-2', which is the preferred grain
growth promotion seed layer, and the first CoFeB ferromagnetic reference
layer 806, 806'. This formation of single-layer-structured reference
layer is explained by the crystallization of the first CoFeB
ferromagnetic reference layer 806, 806' based on the second CoFe
ferromagnetic reference layer 807-2, 807-2' as the preferred grain growth
promotion seed layer. Using the same length reference used in FIG. 21,
the inter-atomic spacing of single-layer-structured CoFe reference layer
after annealing was measured by averaging 7 atomic planes from the
boxed-area in FIG. 22B, which provides the inter-atomic spacing is 2.0
Å. Also the inter-atomic spacing of MgO tunnel barrier was measured
to be 2.13 Å by the length reference. Those inter-atomic spacings
from the single-layer-structured CoFe reference layer and the MgO tunnel
barrier after annealing confirm that both of MgO tunnel barrier and CoFe
reference layer are fully crystalline with (001) out-of-plane texture.
Furthermore, Selected-area diffraction pattern shown in FIG. 22B from
boxed-area in FIG. 22A by fast Fourier transformation using Gatan
Digitalmicrograph confirms the grain-to-grain pseudo-epitaxy, which is
45° rotational epitaxy, between the MgO tunnel barrier and the
CoFe reference layer in that the [001] crystalline axis of single-layered
CoFe reference layer is parallel to [011] crystalline axis of MgO tunnel
barrier. Note that the diffraction patterns indexed with underline are
from single-layer-structured CoFe reference layer and the diffraction
patterns without underline are from MgO tunnel barrier. This
grain-to-grain pseudo-epitaxy in CoFe/MgO/CoFe-based magnetic tunnel
junction is critical pre-requisite to obtain the giant TMR as explained
by Choi et al. in J. Appl. Phys. 101, 013907 (2007).

[0134] With reference to FIG. 10A and FIG. 10B, the magnetotransport
properties of the MTJ prepared by the method of the present invention
were measured using CIPT method. In order for comparison, the MTJ with
MgO tunnel barrier prepared by rf sputtering and the MTJ with MgO tunnel
barrier prepared by same oxidation method, in both of which crystalline
CoFe PGGP seed layer is not employed, are shown as reference. As shown in
FIG. 10A, with reference to the MR ratio and R×A product obtained
from the MTJ with MgO prepared by same oxidation method but without the
insertion of the crystalline PGGP seed layer, it is apparent that the MTJ
which employs the crystalline PGGP layer shows much improved
magnetotranport properties, in that noticeable increase of MR ratio is
obtained with significant reduction of R×A product, which is
comparable to or even better than those from MTJ with MgO tunnel barrier
prepared by rf sputtering. At given R×A product of about 6
Ωμm2, MTJ with MgO prepared by natural oxidation without
the crystalline PGGP seed layer provides MR ratio of 74%, whereas the
MTJs with MgO deposited by natural oxidation with the crystalline CoFe
PGGP seed layer, A stack and B stack, provide 170% and 183%,
respectively, which are comparable to or even higher than 170% obtained
from the MTJ with MgO tunnel barrier prepared by rf sputtering without
the crystalline PGGP seed layer. Clear evidence of R×A product
reduction and MR ratio increase is shown in FIG. 10B. Only difference
between reference and A and B is whether the MTJ employs the crystalline
CoFe PGGP seed layer or not. The natural oxidation condition for all
samples are identical; oxygen flow rate 700 sccm and exposure time 30
seconds. By use of the crystalline PGGP seed layer, R×A product
drops from 7 Ωμm2 to 4.8 Ωμm2, which roughly
drops by two thirds, and MR ratio increases from 74.4% to 169%.

[0135] Furthermore, another set of MTJs with A stack, of which thickness
of the said second crystalline ferromagnetic CoFe reference layer varies,
to optimize the thickness ratio between the first amorphous ferromagnetic
CoFeB reference layer and the second crystalline CoFe reference layer in
the bi-layer-structured ferromagnetic reference layer. Thickness of the
first amorphous CoFeB ferromagnetic reference layer in the
bi-layer-structured ferromagnetic reference layer is fixed at 1.5 nm. As
shown in FIG. 11, it is clearly shown that MR ratio drops sharply and
R×A product increases when the thickness of the second crystalline
CoFe ferromagnetic reference layer is 2.0 nm and over. Thus it can be
concluded that when the thickness of the first amorphous CoFeB
ferromagnetic reference layer of the bi-layer-structured ferromagnetic
reference layer is fixed at 1.5 nm, the thickness of the second
crystalline CoFe ferromagnetic reference layer cannot exceed 1.5 nm to
obtain higher MR ratio at lower R×A product.

[0136] Again, based on the previous studies correlating magnetotransport
property and the crystallinity and pseudo-epitaxy in MTJ, it can be
intuitively inferred that the insertion of the crystalline CoFe PGGP seed
layer adjacent the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, induces the
crystallization and the preferred grain growth of the MgO tunnel barrier
during the post-deposition annealing.

Third Embodiment

[0137] The third embodiment is a method of forming the tunnel barrier of
MTJ devices by use of surfactant layer and followed by radical oxidation
method and the employment of CoFe as the preferred grain growth promotion
seed layer, in that the core element of the MTJ is formed by the
combination of (a+e+i+k) of the group 1, 2, 3 and 4 mentioned above.

[0138] The following configuration of MTJ stack, as illustrated in FIG.
8C, is used in the third embodiment;

[0139] Referring to FIG. 8c, one of important aspect of the third
embodiment is the insertion of the preferred grain growth promotion
(PGGP) seed layer, which is the second crystalline ferromagnetic
reference layer 807-2'', under the MgO tunnel barrier.

deposition of first metallic Mg layer 808'' on the second crystalline
ferromagnetic reference layer 807-2'', which is the preferred grain
growth promotion seed layer, with thickness of 0.43 nm, formation of the
oxygen surfactant layer 814'' within the vacuum chamber by exposing the
0.43 nm of metallic Mg layer 808'' to the oxygen ambience, wherein the
exposure is controlled to be 30 Langmuir by the exposure time and the
oxygen flow through the chamber, deposition of second metallic Mg layer
815'' on the oxygen surfactant layer with thickness of 0.67 nm, oxidation
of the first and second metallic layers 808'' and 815'' by radical
oxidation carried out in the oxidation chamber, in which
electrically-ground "shower plate" is placed between an upper ionizing
electrode and the substrate. Oxygen plasma is generated by applying 300 W
of rf power to the ionizing electrode with oxygen flow of 700 sccm.
Oxygen radical shower flows through the shower plate, whereas particles
with electric charge, such as ionized species and electrons, cannot pass
through due to the electric grounding of shower plate, and deposition of
the metallic Mg cap layer 810'' with thickness of 0.3 nm on the first and
the second metallic Mg layers 808'' and 815'' oxidized by radical
oxidation.

[0143] Post-deposition magnetic field annealing is carried out at
360° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer 806'' and the amorphous ferromagnetic free
layer 811'' and the preferred grain growth of the said amorphous or
microcrystalline MgO tunnel barrier with poor (001) out-of-plane texture.
This crystallization and the preferred grain growth are realized using
the second crystalline CoFe ferromagnetic reference layer as adjacent
crystallization or preferred grain growth seed layer during annealing,
thus eventually overall (001) out-of-plane texture of the ferromagnetic
reference layer, MgO tunnel barrier and the ferromagnetic free layer.

[0144] With reference to FIG. 12, clear evidence of R×A product
reduction and MR ratio increase is confirmed. Only difference between
reference and A is whether the MTJ employs the crystalline PGGP seed
layer or not. The radical oxidation condition for all samples are
identical; 300 W, 700 sccm and 10 seconds. By use of the crystalline PGGP
seed layer, R×A product drops from 22.5 Ωμm2 to 8.3
Ωμm2, which roughly drops by a third, and MR ratio
increases from 120% to 170%.

[0145] Again, based on the previous studies correlating magnetotransport
property and the crystallinity and pseudo-epitaxy in MTJ, it can be
intuitively inferred that the insertion of the crystalline CoFe PGGP seed
layer adjacent the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, induces the
crystallization and the preferred grain growth of the MgO tunnel barrier
during the post-deposition annealing.

Forth Embodiment

[0146] The forth embodiment is a method of forming the tunnel barrier of
MTJ devices by reactive sputtering and the employment of CoFe as the
preferred grain growth promotion seed layer, in that the core element of
the MTJ is formed by the combination of (a+h+i+k) of the group 1, 2, 3
and 4 mentioned above.

[0147] The following configuration of MTJ stacks, as illustrated in FIG.
8D, is used in the forth embodiment;

[0148] Referring to FIG. 8D, one of important aspect of the forth
embodiment is the insertion of the preferred grain growth promotion seed
layer, which is the second crystalline CoFe ferromagnetic reference layer
807-2''', under the MgO tunnel barrier.

deposition of first metallic Mg layer 808''' on the second crystalline
ferromagnetic reference layer 807-2''', which is the preferred grain
growth promotion seed layer, with thickness of 0.6 nm, formation of MgO
layer 816''' through the reactive sputtering of Mg in the mixed gas of
argon and oxygen by flowing argon at the flow rate of 40 sccm and the
oxygen at the flow rate of 4 sccm with thickness of 0.6 nm on the first
metallic Mg layer 808'''. oxidation of the first metallic layer 808'''
and the MgO layer 810''' by natural oxidation carried out in the
oxidation chamber. The natural oxidation process, which is advantageously
applied to the thinly formed metallic Mg layer and the MgO layer,
requires purging the oxidation chamber with oxygen gas at a pressure of
approximately 6.5×10-1 Pa and flowing the oxygen gas at the
flow rate of 700 sccm, then leaving the as-deposited metallic Mg layer
and the MgO layer in contact with the oxygen gas flow for given exposure
time, and deposition of the metallic Mg cap layer 810''' with thickness
of 0.3 nm on the MgO layer and the first metallic Mg layer oxidized by
natural oxidation.

[0152] Post-deposition magnetic field annealing is carried out at
360° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer 806''' and the amorphous ferromagnetic free
layer 811''' and the preferred grain growth of the said amorphous or
microcrystalline MgO tunnel barrier with poor (001) out-of-plane texture.
This crystallization and the preferred grain growth are realized using
the second crystalline ferromagnetic reference layer 807-2''' as adjacent
crystallization or preferred grain growth seed layer during annealing,
thus eventually overall (001) out-of-plane texture of the ferromagnetic
reference layer, MgO tunnel barrier and the ferromagnetic free layer.

[0153] With reference to FIG. 13, clear evidence of R×A product
reduction and MR ratio increase is confirmed. Only difference between
reference and A is whether the MTJ employs the crystalline CoFe PGGP seed
layer or not. The natural oxidation condition for all samples are
identical; oxygen flow rate 700 sccm and exposure time 30 seconds. By use
of the crystalline PGGP seed layer, R×A product drops from 16.5
Ωμm2 to 10.2 Ωμm2 and MR ratio increases from
135% to 185%.

[0154] Again, based on the previous studies correlating magnetotransport
property and the crystallinity and pseudo-epitaxy in MTJ, it can be
intuitively inferred that the insertion of the crystalline CoFe PGGP seed
layer adjacent the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, induces the
crystallization and the preferred grain growth of the MgO tunnel barrier
during the post-deposition annealing.

Fifth Embodiment

[0155] The fifth embodiment is a method of forming the tunnel barrier of
MTJ devices by natural oxidation method and the employment of CoFeB as
the crystalline preferred grain growth promotion seed layer, in that the
core element of the MTJ is formed by the combination of (b+d+j+k) of the
group 1, 2, 3 and 4 mentioned above.

[0156] Two identical configurations of MTJ stacks, as illustrated in FIG.
14, are used in the fifth embodiment with different boron content in the
CoFeB PGGP seed layer as follows;

[0157] Referring to FIG. 14, one of important aspect of the fifth
embodiment is the insertion of the preferred grain growth promotion seed
layers, which are the second crystalline ferromagnetic reference layer
and the first crystalline ferromagnetic free layer, sandwiching the MgO
tunnel barrier.

[0158] With respect to FIG. 15A and FIG. 15B, it is clear that the
microstructure of CoFeB with boron content of 5.1 atomic % and 2.9 atomic
% is crystalline in as-deposited state, whereas that of CoFeB with boron
content of 20 atomic % is amorphous, which is confirmed by xRD theta-2
theta scan from the CoFeB single layer deposited on the thermally
oxidized Si wafer. Intensity is normalized by thickness of CoFeB single
film. Calculated grain size using Sherrer formula shows that the grain
size of CoFeB (B: 2.9 atomic %) is larger than that of CoFeB (B: 5.1
atomic %), which can be reassured by the significant reduction of
resistivity. Resistivity of the CoFeB changes noticeably with its
crystallinity corresponding to its boron contents. Furthermore, the shift
of the XRD peak from 45.35° of CoFeB (B: 2.9 atomic %) to
45.02° of CoFeB (B: 5.1 atomic %) indicates the lattice expansion
of CoFe with its inclusion of boron at the interstitial sites of
body-centered-cubic structure.

[0160] The second crystalline ferromagnetic reference layer in the stack A
of the fifth embodiment, which is the preferred grain growth promotion
seed layer, is deposited by co-sputtering of Co(70 at. %)Fe(30 at. %)
target and Co(60 at. %)Fe(20 at. %)B(20 at. %) target and the composition
ratio is controlled by the manipulation of the power ratio of
co-sputtering. The composition of the second crystalline ferromagnetic
reference layer, which is the PGGP seed layer, is Co(69.9 at. %)Fe(27.2
at. %)B(2.9 at. %) with thickness of 1.5 nm on the first amorphous
ferromagnetic reference layer. With reference to FIG. 15A and FIG. 15B,
it is confirmed that the as-grown state of Co(69.9 at. %)Fe(27.2 at.
%)B(2.9 at. %) layer is crystalline. Also the crystallinity of Co(69.9
at. %)Fe(27.2 at. %)B(2.9 at. %) is confirmed by the reduction of
resistivity to 20.6 μΩ-cm from 111 μΩ-cm of Co(60 at.
%)Fe(20 at. %)B(20 at. %) due to the better crystallinity compared to the
amorphous phase.

[0161] The second crystalline ferromagnetic reference layer in the stack B
of the fifth embodiment, which is the preferred grain growth promotion
seed layer, is deposited by co-sputtering of Co(70 at. %)Fe(30 at. %)
target and Co(60 at. %)Fe(20 at. %)B(20 at. %) target and the composition
ratio is controlled by the manipulation of the power ratio of
co-sputtering. The composition of the second crystalline ferromagnetic
reference layer, which is the PGGP seed layer, is Co(69.3 at. %)Fe(25.6
at. %)B(5.1 at. %) with thickness of 1.5 nm on the first amorphous
ferromagnetic reference layer. With reference to FIG. 15, it is confirmed
that the as-grown state of Co(69.3 at. %)Fe(25.6 at. %)B(5.1 at. %) layer
is crystalline. Also the crystallinity of Co(69.3 at. %)Fe(25.6 at.
%)B(5.1 at. %) is confirmed by the reduction of resistivity to 43.2
pΩ-cm from 111 μΩ-cm of Co(60 at. %)Fe(20 at. %)B(20 at.
%) due to the better crystallinity compared to the amorphous phase.

[0162] The method of forming the MgO tunnel barrier is as follows;

deposition of first metallic Mg layer on the second crystalline
ferromagnetic reference layer, which is the preferred grain growth
promotion seed layer, with thickness of 1.1 nm, oxidation of the first
metallic layer by natural oxidation carried out in the oxidation chamber.
The natural oxidation process, which is advantageously applied to the
thinly formed metallic Mg layer, requires purging the oxidation chamber
with oxygen gas at a pressure of approximately 6.5×10-1 Pa and
flowing the oxygen gas at the flow rate of 700 sccm, then leaving the
as-deposited metallic Mg layer in contact with the oxygen gas flow for
given exposure time, and deposition of the metallic Mg cap layer with
thickness of 0.3 nm on the first metallic Mg layer oxidized by natural
oxidation.

[0163] The first crystalline ferromagnetic free layer in the stack A of
the fifth embodiment, which is the preferred grain growth promotion seed
layer, is deposited by co-sputtering of Co(70 at. %)Fe(30 at. %) target
and Co(60 at. %)Fe(20 at. %)B(20 at. %) target and the composition ratio
is controlled by the manipulation of the power ratio of co-sputtering.
The composition of the second crystalline ferromagnetic reference layer,
which is the PGGP seed layer, is Co(69.9 at. %)Fe(27.2 at. %)B(2.9 at. %)
with thickness of 1.5 nm on the metallic Mg cap layer.

[0164] The first crystalline ferromagnetic free layer in the stack B of
the fifth embodiment, which is the preferred grain growth promotion seed
layer, is deposited by co-sputtering of Co(70 at. %)Fe(30 at. %) target
and Co(60 at. %)Fe(20 at. %)B(20 at. %) target and the composition ratio
is controlled by the manipulation of the power ratio of co-sputtering.
The composition of the second crystalline ferromagnetic reference layer,
which is the PGGP seed layer, is Co(69.3 at. %)Fe(25.6 at. %)B(5.1 at. %)
with thickness of 1.5 nm on the metallic Mg cap layer.

[0165] Then the second amorphous Co(60 at. %)Fe(20 at. %)B(20 at. %)
ferromagnetic free layer with thickness of 1.5 nm is deposited on the
first crystalline ferromagnetic free layer, which is the preferred grain
growth promotion seed layer.

[0166] Post-deposition magnetic field annealing is carried out at
360° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer and/or the second amorphous ferromagnetic
free layer and the preferred grain growth of the said amorphous or
microcrystalline MgO tunnel barrier with poor (001) out-of-plane texture.
This crystallization and the preferred grain growth are realized using
the second crystalline ferromagnetic reference layer and/or the first
crystalline ferromagnetic free layer as adjacent crystallization or
preferred grain growth seed layer during annealing, thus eventually
overall (001) out-of-plane texture of the ferromagnetic reference layer,
MgO tunnel barrier and the ferromagnetic free layer.

[0167] With reference to FIG. 16A and FIG. 16B, the magnetotransport
properties of the MTJ prepared by the method of the present invention
were measured using CIPT method. In order for comparison, the MTJ with
MgO tunnel barrier prepared by rf sputtering and the MTJ with MgO tunnel
barrier prepared by same oxidation method, in both of which crystalline
PGGP seed layer is not employed, are shown as reference. As shown in FIG.
16A, with reference to the MR ratio and R×A product obtained from
the MTJ with MgO prepared by same oxidation method but without the
insertion of the crystalline PGGP seed layer, it is apparent that the MTJ
employs the crystalline CoFeB PGGP layer shows much improved
magnetotranport properties, in that noticeable increase of MR ratio is
obtained with significant reduction of R×A product, which is
comparable to or even better than those from MTJ with MgO tunnel barrier
prepared by rf sputtering. At given R×A product of about 6
Ωμm2, MTJ with MgO prepared by natural oxidation without
the crystalline PGGP seed layer provides MR ratio of 74%, whereas the
MTJs with MgO deposited by natural oxidation with the crystalline CoFeB
PGGP seed layer, A stack and B stack, provide 178% and 170%,
respectively, which are comparable to or even higher than 170% obtained
from the MTJ with MgO tunnel barrier prepared by rf sputtering without
the crystalline PGGP seed layer. Clear evidence of R×A product
reduction and MR ratio increase is shown in FIG. 16B. Only difference
between reference and A and B is whether the MTJ employs the crystalline
PGGP seed layer or not. The natural oxidation condition for all samples
is identical; oxygen flow rate 700 sccm and exposure time 30 seconds. By
use of the crystalline PGGP seed layer, R×A product drops from 7
Ωμm2 to 4.2 Ωμm2 and MR ratio increases from
74.4% to 160%.

[0168] Again, based on the previous studies correlating magnetotransport
property and the crystallinity and pseudo-epitaxy in MTJ, it can be
intuitively inferred that the insertion of the crystalline PGGP seed
layer adjacent the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, induces the
crystallization and the preferred grain growth of the MgO tunnel barrier
during the post-deposition annealing.

Sixth Embodiment

[0169] The sixth embodiment is a method of forming the tunnel barrier of
MTJ devices by natural oxidation method and the employment of Fe as the
crystalline preferred grain growth promotion seed layer, in that the core
element of the MTJ can be formed by the combination of (c+d+j+k) of the
group 1, 2, 3 and 4 mentioned above.

[0170] The following configuration of MTJ stack, as illustrated in FIG.
17, can be used in the sixth embodiment;

[0171] Referring to FIG. 17, one of important aspect of the sixth
embodiment is the insertion of the preferred grain growth promotion seed
layer, which is the second crystalline ferromagnetic Fe reference layer
1707 and the first crystalline ferromagnetic Fe free layer 1711,
sandwiching the MgO tunnel barrier (1708, 1709, 1710).

[0173] The second crystalline ferromagnetic Fe reference layer 1707 in the
stack A of the sixth embodiment, which is the preferred grain growth
promotion seed layer, is deposited with thickness of 0.5-2 nm on the
first amorphous ferromagnetic reference layer 1706.

[0174] The method of forming the MgO tunnel barrier is as follows;

deposition of first metallic Mg layer 1708 on the second crystalline
ferromagnetic reference layer 1707, which is the preferred grain growth
promotion seed layer, with thickness of 1.1 nm, oxidation of the first
metallic layer 1708 by natural oxidation carried out in the oxidation
chamber. The natural oxidation process, which is advantageously applied
to the thinly formed metallic Mg layer, requires purging the oxidation
chamber with oxygen gas and flowing the oxygen gas, then leaving the
as-deposited metallic Mg layer in contact with the oxygen gas flow for
given exposure time, and deposition of the metallic Mg cap 1710 layer
with thickness of 0.3 nm on the first metallic Mg layer 1709 oxidized by
natural oxidation.

[0175] The first crystalline ferromagnetic Fe free layer 1711 in the stack
A of the sixth embodiment, which is the preferred grain growth promotion
seed layer, is deposited with thickness of 0.5-2 nm on the metallic Mg
cap layer 1710.

[0176] Then the second amorphous Co(60 at. %)Fe(20 at. %)B(20 at. %)
ferromagnetic free layer 1712 with thickness of 1-4 nm is deposited on
the first crystalline ferromagnetic free layer 1711, which is the
preferred grain growth promotion seed layer.

Seventh Embodiment

[0177] The seventh embodiment is a method of forming the tunnel barrier of
MTJ devices by natural oxidation method and the employment of CoFe as the
preferred grain growth promotion seed layer and also the employment of
bi-layer structured pinned layer, in that the core element of the MTJ is
formed by the combination of (a+d+i+l) or (a+d+j+l) of the group 1, 2, 3
and 4 mentioned above.

[0178] The configuration of MTJ stacks, as illustrated in FIG. 18, is used
in the seventh embodiment as follows;

[0179] Referring to FIG. 18, one of important aspect of the seventh
embodiment is the insertion of the preferred grain growth promotion seed
layer, which is the second crystalline CoFe ferromagnetic reference layer
1805 and/or the first crystalline CoFe ferromagnetic free layer 1812,
under or sandwiching the MgO tunnel barrier (1809, 1810, 1811). Another
important aspect of the seventh embodiment is the employment of the
bi-layer-structured ferromagnetic pinned layer, in that the first
amorphous CoFeB ferromagnetic pinned layer 1804 on the antiferromagnetic
PtMn pinning layer 1803 and the second crystalline CoFe ferromagnetic
pinned layer 1805 on the first amorphous CoFeB ferromagnetic pinned layer
1804 are deposited with the thickness of 1.25 nm and 1.25 nm,
respectively.

deposition of first metallic Mg layer 1809 on the second crystalline
ferromagnetic reference layer 1808, which is the preferred grain growth
promotion seed layer, with thickness of 0.7 nm, oxidation of the first
metallic layer 1809 by natural oxidation carried out in the oxidation
chamber. The natural oxidation process, which is advantageously applied
to the thinly formed metallic Mg layer, requires purging the oxidation
chamber with oxygen gas at a pressure of approximately
9.9×10-2 Pa and flowing the oxygen gas at the flow rate of 100
sccm, then leaving the as-deposited metallic Mg layer in contact with the
oxygen gas flow forgiven exposure time, and deposition of the metallic Mg
cap layer 1811 with thickness of 0.3 nm on the first metallic Mg layer
1810 oxidized by natural oxidation.

[0183] Post-deposition magnetic field annealing is carried out at
380° C. for 2 hour under 10 kOe magnetic field. The purposed of
post-deposition annealing is the crystallization of the first amorphous
ferromagnetic reference layer 1807 and/or the second amorphous
ferromagnetic free layer 1813 and the preferred grain growth of the
amorphous or microcrystalline MgO tunnel barrier with poor (001)
out-of-plane texture. This crystallization and the preferred grain growth
are realized using the second crystalline CoFe ferromagnetic reference
layer 1805 and/or the first crystalline CoFe ferromagnetic free layer
1812 as adjacent crystallization or preferred grain growth seed layer
during annealing, thus eventually overall (001) out-of-plane texture of
the ferromagnetic reference layer, MgO tunnel barrier and the
ferromagnetic free layer.

[0184] With reference to FIG. 19, the magnetotransport properties of the
MTJ prepared by the method of the present invention were measured using
CIPT method. In order for comparison, the MTJ with the structure of
"Bottom layers/PtMn15/CoFe2.5/Ru0.9/CoFeB3/Mg1.1/N--Ox
seconds/Mg0.3/CoFeB3/Capping layers", in which crystalline CoFe PGGP seed
layer is not employed and the ferromagnetic pinned layer is CoFeB single
layer, is shown as reference (marked .box-solid.). Also the reference MTJ
is annealed at 360° C. for 2 hours under 10 kOe magnetic field. As
shown in FIG. 19, with reference to the MR ratio and R×A product
obtained from the MTJ with MgO prepared by same oxidation method but
without the insertion of the crystalline PGGP seed layer, it is apparent
that the MTJ which employs the crystalline PGGP layer shows much improved
magnetotranport properties, in that noticeable increase of MR ratio is
obtained with significant reduction of R×A product. At given
R×A product of about 1.5 Ωμm2, MTJ with MgO prepared
by natural oxidation without the crystalline PGGP seed layer provides MR
ratio of 25.8%, whereas the MTJ with MgO deposited by natural oxidation
with the crystalline CoFe PGGP seed layer, A stack, provides 168.8%.

[0185] Again, based on the previous studies correlating magnetotransport
property and the crystallinity and pseudo-epitaxy in MTJ, it can be
intuitively inferred that the insertion of the crystalline CoFe PGGP seed
layer adjacent the MgO tunnel barrier, which is amorphous or
microcrystalline with poor (001) out-of-plane texture, induces the
crystallization and the preferred grain growth of the MgO tunnel barrier
during the post-deposition annealing.

[0187] This enhanced thermal stability of magnetic tunnel junction with
`bi-PL` can be explained by the boron segregation blocking manganese (Mn)
diffusion from antiferromagnetic Mn-alloy. FIG. 26 shows the schematic
explanation of enhanced thermal stability. During annealing, boron in
CoFeB pinned layer diffuses out while CoFeB crystallizes based on CoFe
pinned layer as crystallization template and segregates at the
CoFeB/Mn-alloy interface. As Mn diffusion has been reported to be
responsible for the reduction of Hex, Mn diffusion barrier, which is
boron segregated at the interface, is attributed to achieving the better
thermal stability. This thermal stability of magnetic tunnel junction
enlarges the process window for MRAM production as it includes high
temperature CMOS process.

[0188] The present invention includes an embodiment wherein only a
ferromagnetic pinned layer deposition step comprises a crystalline
ferromagnetic pinned layer formation step and an amorphous ferromagnetic
pinned layer formation step. And the crystalline ferromagnetic pinned
layer is nearer to a non-magnetic spacer layer than the amorphous
ferromagnetic pinned layer.

Patent applications by Yuichi Otani, Kawasaki-Shi JP

Patent applications by CANON ANELVA CORPORATION

Patent applications in class HAVING MAGNETIC OR FERROELECTRIC COMPONENT

Patent applications in all subclasses HAVING MAGNETIC OR FERROELECTRIC COMPONENT